test/cctest/compiler/value-helper.h: fix due to a GCC bug affecting padding
of aggregate data types on Aix.

GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61976

Change-Id: I303b09a9a84b372033c16cbf157d5fc2bf25f93a
Reviewed-on: https://chromium-review.googlesource.com/1244616Reviewed-by: Daniel Clifford <danno@chromium.org>
Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#56274}